Silicon wafer edges are very frequently damaged during shipping, handling and processing. For example, mechanical damage from wafer handling can cause cracks, which can lead to wafer breakage, as well as chips that result in the creation of particles. Thermal cycling can cause delamination of thin films or plastic deformation of the wafer through slip, which can lead to problems with overlay alignment required for advanced lithography. These sorts of wafer-edge defects, if undetected, can have catastrophic consequences, and may lead to loss of an entire wafer in late stages of processing.
Various methods have been developed for detection of wafer-edge defects. For example, U.S. Pat. No. 6,062,084 describes an apparatus in which an ultrasonic detection unit is used to detect crazing or micro-cracks in a wafer edge, while a laser detection unit is used for detecting cracks in the wafer edge. As another example, U.S. Pat. No. 7,508,504 describes an automatic wafer edge inspection and review system, in which a light diffuser with a plurality of lights provides uniform diffuse illumination of a substrate. An optic and imaging system exterior of the light diffuser is used to inspect the plurality of surfaces of the substrate including specular surfaces. The optic can be rotated radially relative to a center point of the substrate edge to allow for focused inspection of all surfaces of the substrate edge.
X-ray diffraction imaging (XRDI), also known as X-ray topography, has been used to detect crystalline defects based on local changes in the diffracted X-ray intensity. For example, U.S. Pat. No. 6,782,076, whose disclosure is incorporated herein by reference, describes an X-ray topographic system, comprising an X-ray generator producing a beam of X-rays impinging on a limited area of a sample such as a silicon wafer. A solid-state detector is positioned to intercept the beam after transmission through or reflection from the sample and produces a digital image of the area on which the X-rays impinge. Relative stepping motion between the X-ray generator and the sample produces a series of digital images, which are combined together. In optional embodiments, an X-ray optic is interposed to produce a parallel beam to avoid image doubling, or the effect of image doubling is removed by software.